Is it not better to use "well-rounded" in: They are a well-rounded team and have pretty good chances of winning the championship. I head "all rounded" used instead, but according to my dictionary "well-rounded" sounds better. Is that true?

Thank you.
Yes. 'Well-rounded' is grammatically correct.
An 'all-round performance' is okay; the team can be full of 'all-rounders' but the performance cannot be 'all-rounded'.
Students: Are you brave enough to let our tutors analyse your pronunciation?
Thank you.

Actually, I seriously do not know. But I think that both are fine.